| Due to external loading,the negative environment effect and other unfavorable influence factors,the damage accumulation and reduction of the load Load carrying capacity of concrete in sevice may take place.The influence above mentioned will cause the expansion of concrete cracks and the formation of new structure cracks,reduce the serviceability and durability significantly,accelerate the aging of the structure and even lead to sudden failure of concrete.Therefore,it is necessary to track the concrete cracks in order to achieve real-time monitoring and judgement of the structural health status.Compared to other types of sensor,a new kind of intelligent structural materials which can self-monitoring both the crack extension,loading change and improve the toughness as well as other mechanical properties should be developed.An integration of sensor and structure member could be explored and realized,in order to provide a new way and thinking method for health monitoring of brdge and other concrete structures.The main contents are as follows:(1)The effect of macro steel fiber,carbon fiber and nano-carbon black conductive phases on the workability of fresh concrete were compared.The results showed that the higher the content of conductive phases,the higher gas content,lower slump and worse workability of concrete mixture were achieved.(2)The effect of macro steel fiber,carbon fiber and nano-carbon black on the compressive strength and flexural toughness of concrete were studied.The minimum fiber content leading to deflection hardening was determined.The results showed that the addition of conductive materials has little effect on the compressive strength.The higher the steel fiber content,the better the flexural toughness of the concrete was achieved.The concrete with 60kg/m3 steel fiber appeared deflection hardening.The influence of the carbon fiber and nano-carbon black on flexural toughness was not obvious.(3)The effect of conductive materials on the sensitivity of cracking development for smart concrete under bending were compared.The relative optimal conductive materials were determined according to the signal-to-noise ratio.The results showed that the fractional change in surface resistance of the smart concrete increased with the expansion of the deflection/ crack width,and could sense the loss of load carrying capacity.Compared to the single-phase smart concrete,the biphasic and triphasic smart concrete had higher signal-to-noise ratio.Taking the cost factor into consideration,it was determined that the steel fiber and nano-carbon black was the relative optimal combination of conductive materials.(4)The relationship between the fractional change in resistance and the midspan deflection/crack width was compared when the deflection hardening and softening happened.The stage model of the curve and the analysis model of fractional change in resistance was established.The results showed that the deflection softening can be divided into two stages,and the deflection hardening can be divided into three stages,corresponding to different Load carrying capacity/crack morphology change process.It could be seen from the analysis model that the fractional change in resistance was affected by the crack propagation and the strain of uncracked matrix.(5)The influence of temperature,humidity and test frequency on the resistance and capacitance of the biphasic smart concrete was studied.The results showed that the resistance increased and the capacitance decreased with increasing water loss.The resistance decreased and the capacitance increased with increasing temperature.The resistance of the specimen was exponentially decaying and the activation energy of the smart concrete showed a decreasing trend with inceasing test frequency. |
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